The invention relates to the use of macrocyclic metal-ligand complexes as bleaching catalysts, and more particularly, to transition metal complexes of macrocyclic tetraamide ligands as catalysts for enhancing oxidative bleaching reactions.
The United States and Canada are the world's leading producers of wood pulp used for the production of paper and paper board. In 1994, the United States produced over 58 million tons of wood pulp. Pulp, which is made either mechanically or chemically from wood, contains 1) cellulose, a homopolysaccharide linear polymer of d-glucose of the formula --(C.sub.6 H.sub.10 O.sub.5)--; 2) lignin, a nonuniform three dimensional molecule having the following general composition, C.sub.9 H.sub.8.83 O.sub.2.37 (OCH.sub.3).sub.0.96 ; and 3) hemicellulose, a heteropolysaccharide polymer. See generally, W. G. Glasser and S. Sarkanen, eds. "LIGNIN PROPERTIES AND MATERIALS," American Chemical Society Symposium, Series 397.
Desirable qualities for paper include strength, whiteness and brightness. The strength of the paper is related to the viscosity of the pulp employed in its manufacture which, in turn, is related to the condition of the cellulose after the pulping operation. Molecular cellulose, as explained above, is a linear chain of d-glucose which naturally forms long fibers. The longer the individual cellulose polymer chains, the higher the viscosity of the pulp, and in turn, the greater the strength of the paper. Thus, during processing, it is most desirable to avoid cleaving the cellulose polymers into smaller units.
Whiteness is based on the appearance of the paper to observers and its measure is therefore subjective. Brightness is a term used to describe the whiteness of pulp on a scale from 0% (absolute black) to 100% (relative to a MgO standard, which has an absolute brightness of ca. 96%) by the reflectance of blue light (457 nm) from the paper produced from the pulp. The more incident light that is reflected, rather than absorbed, the brighter the paper.
Brightness is obtained by bleaching. Pulp bleaching is defined as the treatment of cellulose fibers with chemicals to increase brightness. Bleaching chemicals increase brightness by removing and decolorizing the lignin in the pulp. Lignin exhibits a yellowish to a deep brown color, depending on the type of wood.
The most common bleaching chemicals are the oxidants chlorine, a source of hypochlorite ion, and chlorine dioxide. Oxygen gas in conjunction with NaOH may also be used, but requires expensive equipment and must be used in large amounts. Oxygen also results in loss of pulp strength resulting from free radical damage to the cellulose polymers, particularly when the lignin content of the pulp is low.
Chlorine and hypochlorite can result in loss of strength if used improperly, but in general are effective and relatively easy to use oxidants. Hypochlorite is an aggressive oxidant that is prone to attacking the cellulose, especially if nonoptimally employed. Chlorine dioxide achieves a high level of brightness without pulp degradation. However, it is an expensive oxidant and it is prone to explosive decomposition. All the chlorine based oxidants produce as effluent chlorinated byproducts that are hazardous to the environment and to health. Moreover, effluent that contains chlorine in any chemical form cannot be burned in the recovery boiler of a pulp mill. The chlorine produces corrosion of the recovery boiler. Moreover, as noted below, combustion of chlorine containing species can lead to the production of polychlorinated dioxins and dibenzofurans, 17 of which are considered toxic and carcinogenic. In addition, chlorine, for example, can react violently with combustible materials. It reacts with H.sub.2 S, CO and SO.sub.2 to form toxic and corrosive gases; and, in liquid form, causes burns, blistering and tissue destruction. In gaseous form, it causes severe irritation to eyes, nasal passages and respiratory tissue. In high doses, it can be lethal. Chlorine dioxide bleach decomposes into Cl.sub.2 which is toxic and corrosive.
Polychlorinated aromatic compounds are environmental pollutants. The most well known examples are DDT, the polychlorinated phenols, dioxins, dibenzofurns and polychlorinated biphenyls (PCBs). These types of compounds can be formed when appropriate organic compounds are exposed to chlorine containing oxidants. The combustion of organic matter in the presence of chlorine in any form can produce dioxin. Even though dioxins and PCBs are no longer manufactured, there are chemical processes that form these compounds from polychlorinated phenol precursors. There is a need to prevent the unwanted formation of polychlorinated aromatic compounds and to remediate those that are present in the environment.
In the pulp and paper industry, chlorinated organics (monochlorinated and polychlorinated), collectively called "absorbable or adsorbable organic halogen" or "AOX", are formed upon bleaching of wood pulp with chlorine based oxidants. One such compound is 2,4,6-trichlorophenol, TCP, which is produced, for example, during the bleaching process when chlorine is used as the bleaching agent. TCP ends up in the waste stream leaving the plant.
Notwithstanding the hazards to the environment, the chlorine-based oxidants are the most widely used for pulp bleaching in the United States. Commercial pulp and paper bleaching facilities actually use a combination of several methods. One widely used bleaching sequence begins with chlorination, followed by extraction with NaOH, treatment with chlorine dioxide, more NaOH extraction and then more chlorine dioxide treatment. A modification of that sequence adds a hypochlorite oxidation step between the first NaOH extraction and first treatment with chlorine dioxide. In another sequence, the second NaOH extraction and second chlorine dioxide treatment are eliminated.
On Nov. 14, 1997, the United States Environmental Protection Agency signed a Cluster Rule requiring the Pulp and Paper Industry to reduce chlorinated organics production. To meet the effluent reduction requirements, the industry is primarily expanding the use of what is called "elemental chlorine free" (ECF), which is a term used primarily for bleaching with chlorine dioxide. The important point is that chlorine dioxide bleaching produces considerably less toxic effluent than does bleaching with elemental chlorine, Cl.sub.2. Nevertheless, some AOX is produced and a further disadvantage is that the bleach plant effluent cannot be burned in the recovery boiler as noted above. In addition, the industry has been developing what is calls "totally chlorine free" (TCF) bleaching. The principal oxidants of TCF bleaching are oxygen and hydrogen peroxide, although ozone also has a position. Hydrogen peroxide oxidizes and brightens lignin and produces high yields of pulp. It is easier to use than oxygen and it does not require expensive equipment, one of the big disadvantages of oxygen bleaching. In use, it is generally believed that H.sub.2 O.sub.2 dissociates to produce the perhydroxyl ion, OOH--, which decolorizes lignin and does not attack cellulose. However, if H.sub.2 O.sub.2 decomposes, it produces free radicals which fragment the lignin as desired, but also degrades the cellulose. The principal offending radical is the hydroxyl radical, HO.cndot. which is notoriously nonselective. Because the H--O bond of water is so strong (ca. 119 kcal.morl.sup.-1), the HO.cndot. radical will abstract H atoms rapidly from a wide variety of organic compounds and, indeed, from most H-atom sources. For this reason, pulp is generally treated with a sequestering agent prior to peroxide treatment. The purpose of the sequestering agent is to remove metal ions which decompose the peroxy compound producing radicals. Furthermore, peroxide bleaching methods will often include the addition of further sequestering agent, again for shielding the peroxy compound from exposure to trace amounts of metal which can decompose it unnecessarily and lower its selectivity. While hydrogen peroxide itself is a strong oxidant which can burn skin and mucous membranes, it is not a serious hazard in low (&lt;8%) concentrations. Most importantly, its use does not introduce elemental toxicity into the environment. Peroxide is an excellent brightening agent. The major drawback to use of H.sub.2 O.sub.2 as the oxidant for pulp and paper bleaching is that it is not as selective at delignifying pulp as chlorine dioxide. The process proceeds relatively slowly such that the pulp and peroxide must be heated. Historically, peroxide was a comparatively expensive oxidant. However, peroxide prices have been falling and mills have the option for onsite generation. Although H.sub.2 O.sub.2 would clearly be preferred for its environmentally friendly characteristics, the selectivity factors and operating costs associated with its use have contributed to reducing its commercial desirability. When used commercially, it has been primarily as a brightening agent for mechanical pulp used, for example, in newsprint when long term stability of the color is not needed and the lignin is primarily decolorized rather than being essentially removed, or as an adjunct to chlorination and/or chlorine dioxide bleaching and/or oxygen bleaching or to oxidize the effluent.
The environmental impacts of wastewaters produced by pulp and paper processing have been the focus of significant research over the past 30 years. The traditional areas of concern have been oxygen demand, suspended solids and acute toxicity. Improvements in control strategies within mills, pulping and bleaching technology, and secondary treatment systems have addressed these issues to a large extent. There is now an increasing focus on potential sub-acute toxicity (e.g. reproductive effects), residual nutrients/eutrophication, and recalcitrant constituents, especially color and organochlorine. Reductions in wastewater color and absorbable organic halogen mass loadings following biological treatment may average 10% and 40%, respectively. In some cases, significant increases in color levels may occur. Approximately 50% of the soluble chemical oxygen demand in bleached kraft mill effluents (BKME) also remains following secondary treatment and appears to consist of recalcitrant high molecular mass material (HMM). The higher molecular mass constituents (MW&gt;1000 Daltons) in BKME consist primarily of highly degraded, chlorinated lignin degradation products with some residual polysaccharide constituents. In BKME, this material may constitute 40-90% of the total organic material, approximately 80% of the AOX content, and 60-100% of the color loadings from the mill. Little information is available on the chemical nature or mass flows of color and HMM discharged from other mill operations (e.g. mechanical pulping).
Scandinavian and North American studies have shown that these recalitrant chromophoric, halogenated and carbonaceous constituents are highly persistent in freshwater systems and may be detected hundreds of kilometers from the discharge source. The colored material has obvious effects on the aesthetics of receiving water as well as decreasing photic depth in the water column and thus the available habitat for macrophytes, and planktonic and benthic food sources. HMM was previously considered to be nonbioaccumulative, non-toxic and inert because of its large molecular size and water solubility. More recent studies now indicate that bioaccumulation of some of this material-can occur in exposed organisms and HMM has been implicated as the major toxicant inhibiting fertilization capacity in some marine species (e.g. echinoderms). HMM may also absorb low molecular weight lipophilic ecotoxicants such as chlorophenolics. This association may significantly effect the dispersal and transportation of these ecotoxicants and alter their bioavailability to recipient organisms.
Given these concerns, the pulp and paper industry is under considerable pressure to effectively remove these wastewater constituents. A large number of advanced technologies exist, in various stages of development, that have potential application for their environmental control. These include ultrafiltration; flocculation; electrotechnologies such as ozonolysis, photolysis, and wet oxidation; and incineration and plasmolysis. Significant potential exists for the application of a number of different technologies to give the desired treatment, such as the use of advanced treatment in conjunction with biological and/or membrane separation technology.
Advanced oxidative technologies, for example using ozone or peroxide, have shown particular promise as highly effective means of removing organochlorine or color from pulp and paper wastewaters. However, the high levels of peroxide required in treatments developed to date would appear to make this technology as prohibitively expensive as the other methods.
Certain transition metal chelates have been researched for unrelated purposes. For example, complexes of high oxidation state transition metals are known to function as oxidants in numerous biological reactions under the influence of a protein matrix and in recent years a widespread interest in understanding the mechanism of action and the reactivity of certain monooxygenase catalysts has developed.
An exemplary program is described in Collins, T. J., "Designing Ligands for Oxidizing Complexes," Accounts of Chemical Research, 279, Vol. 27, No. 9 (1994). This article lays out a design oriented approach for obtaining ligands that are resistant to oxidative degradation when coordinated to highly oxidizing metal centers. Several diamido-N-diphenoxido and diamido-N-alkoxido acyclic chelate compounds and macrocyclic tetraamido-N chelate compounds are described in the Collins Accounts of Chemical Research article.
An azide based synthetic route to macrocyclic tetraamido metal ligand complexes is described in Uffelman, E. S., Ph.D. Thesis, California Institute of Technology, (1992). Additionally, synthesis of an aryl bridged tetraamido ligand via the azide based route can proceed by using an aromatic diamine as a starting material.
However, the art has not recognized that certain macrocyclic tetraamido metal ligand complexes will provide novel and unusually effective bleach activators for peroxy compounds. Additionally, it has not been taught, disclosed or suggested that these types of compounds will be unusually advantageous in the areas of pulp and paper bleaching.